Managing Spectra of Modulated Signals in a Communication Network

ABSTRACT

Information is modulated onto frequency components of a signal. The resulting modulated signal includes at least some redundancy in frequency enabling a portion of the information modulated onto selected frequency components to be recovered from fewer than all of the selected frequency components. Controlling the spectrum of the modulated signal includes enabling the amplitude of at least some frequency components of the modulated signal to be set below a predetermined amplitude used for modulating the information.

PRIORITY INFORMATION

This application is a continuation of U.S. application Ser. No.11/493,382, titled MANAGING SPECTRA OF MODULATED SIGNALS IN ACOMMUNICATION NETWORK, by Lawrence W. Yonge III, which was filed on Jul.26, 2006, which claims priority to U.S. Application Ser. No. 60/702,717,titled POWERLINE NETWORK MANAGEMENT AND CONTROL by Lawrence W. YongeIII, and filed on Jul. 27, 2005, and U.S. Application Ser. No.60/705,720, titled COMMUNICATING IN A NETWORK THAT INCLUDES A MEDIUMHAVING VARYING TRANSMISSION CHARACTERISTICS by Yiorgos M. Peponides etal. and filed on Aug. 2, 2005, each of which is incorporated herein byreference as though fully and completely set forth herein.

TECHNICAL FIELD

The invention relates to managing spectra of modulated signals in acommunication network.

BACKGROUND

Various types of communication systems transmit signals that may radiatein a portion of the electromagnetic spectrum and cause interference withdevices that operate in that portion of the electromagnetic spectrum(e.g., radio frequency (RF) spectral bands). In some cases regulatoryrequirements for certain geographical regions (e.g., imposed bygovernments) place constraints on power that may be radiated in certainspectral regions, such as amateur radio bands. Some systems are wirelesssystems that communicate between stations using radio waves modulatedwith information. Other systems are wired systems that communicate usingsignals transmitted over a wired medium, but the wired medium mayradiate enough power in restricted spectral bands to potentially causeinterference.

Communication stations can be configured to avoid using or limit theamount of power that is radiated in certain restricted spectral bands.Alternatively, communication stations can be configured to adjust thespectral regions used for communication, based on whether the station isoperating in an environment in which interference may occur. Forexample, orthogonal frequency division multiplexing (OFDM), also knownas Discrete Multi Tone (DMT), is a spread spectrum signal modulationtechnique in which the available bandwidth is subdivided into a numberof narrowband, low data rate channels or “carriers.” To obtain highspectral efficiency, the spectra of the carriers are overlapping andorthogonal to each other. Data are transmitted in the form of symbolsthat have a predetermined duration and encompass some number ofcarriers. The data transmitted on these carriers can be modulated inamplitude and/or phase, using modulation schemes such as Binary PhaseShift Key (BPSK), Quadrature Phase Shift Key (QPSK), or m-bit QuadratureAmplitude Modulation (m-QAM). An example of a system in which carrierscan be disabled to avoid potential interference is described in moredetail in U.S. Pat. No. 6,278,685, incorporated herein by reference. Inthis system, after one or more carriers are disabled, the modulationfunctions (e.g., an interleaver shift mechanism) are adjusted for adifferent number of usable carriers.

SUMMARY

In one aspect, in general, the invention features a method that includesmodulating information onto frequency components of a signal. Theresulting modulated signal includes at least some redundancy infrequency enabling a portion of the information modulated onto selectedfrequency components to be recovered from fewer than all of the selectedfrequency components. The method includes controlling the spectrum ofthe modulated signal, including enabling the amplitude of at least somefrequency components of the modulated signal to be set below apredetermined amplitude used for modulating the information.

Aspects of the invention may include one or more of the followingfeatures.

Modulating the portion of the information onto selected frequencycomponents comprises modulating redundant data from which the portion ofthe information can be decoded onto respective frequency componentshaving different center frequencies.

The signal comprises a plurality of symbols, and at least some of therespective frequency components are in different symbols.

The redundant data comprises one or more encoded bits associated withthe information.

The one or more encoded bits comprise data bits representing theinformation.

The one or more encoded bits comprise parity bits used for decoding theinformation.

The center frequencies are spread approximately uniformly over most of aset of frequency components available for modulating the information.

The method further comprises transmitting the modulated signal from afirst node to a second node.

The first node and the second node each stores information identifying aset of frequency components available for modulating the information.

The second node is able to recover the portion of the informationwithout needing to receive information from the first station indicatingwhether any of the selected frequency components have been set below thepredetermined amplitude used for modulating the information.

The method further comprises demodulating each of the selected frequencycomponents, and using resulting demodulated information to recover theportion of the information.

Recovering the portion of the information comprises decoding thedemodulated information.

The amplitude of at least one of the selected frequency components hasbeen set below the predetermined amplitude used for modulating theinformation.

The predetermined amplitude used for modulating the informationcomprises an amplitude corresponding to a phase shift keying modulationconstellation.

The predetermined amplitude used for modulating the informationcomprises one of a plurality of amplitudes corresponding to a quadratureamplitude modulation constellation.

Setting the amplitude of one of the frequency components below thepredetermined amplitude used for modulating the information comprisessetting the amplitude of the frequency component below a limit based ona constraint on power that can be radiated in a portion of the spectrumof the modulated signal in which the frequency component is located.

The constraint on the power is based on a prohibition from interferingwith a licensed entity.

The method further comprises setting the amplitude of the frequencycomponent below the limit in response to detecting a transmission fromthe licensed entity.

Setting the amplitude of one of the frequency components below thepredetermined amplitude used for modulating the information comprisesturning off the frequency component.

The method further comprises selecting the frequency components of thesignal according to a set of available frequencies that excludes atleast some frequencies in a range of frequencies.

The excluded frequencies correspond to frequencies that are likely tointerfere with licensed entities in a region.

In another aspect, in general, the invention features a transmitter. Thetransmitter includes an encoder module including circuitry toredundantly encode information to be modulated onto frequency componentsof a signal, the resulting modulated signal including at least someredundancy in frequency enabling a portion of the information modulatedonto selected frequency components to be recovered from fewer than allof the selected frequency components. The transmitter also includes aspectral shaping module including circuitry to control the spectrum ofthe modulated signal, including enabling the amplitude of at least somefrequency components of the modulated signal to be set below apredetermined amplitude used for modulating the information.

Among the many advantages of the invention (some of which may beachieved only in some of its various aspects and implementations) arethe following.

The amplitude mask technique can be used to preserve interoperabilitybetween a user's local network (e.g., a home powerline network ofdevices such as computer, Ethernet bridge, TV, DVR, etc.) and an accessnetwork of a service provider, for example. The service provider mayneed to limit power radiated in a given spectral band due a constraintsuch as a prohibition from interfering with a licensed entity. TheFederal Communications Commission (FCC) may require that the serviceprovider be able to have a way to stop transmitting power in a givenspectral band if they interfere with a licensed entity such as anamateur radio device or a radio station, for example. The amplitude masktechnique enables the service provider to adjust the transmittedspectrum while preserving communication without the need to negotiate achange in modulation scheme with receiving stations.

For example, if a service provider is already communicating with auser's device using a given set of carriers, and the service providerneeds to turn off one or more of the carriers, the amplitude masktechnique enables the service provider to stop radiating power on aninterfering carrier while still using that carrier in a modulationscheme agreed upon with the user station. Since the amplitude maskchanges the amplitude of selected carriers but does not eliminate thosecarriers from the modulation scheme, the amplitude mask technique avoidsthe communication overhead of updating modulation parameters (e.g., thetone mask) before adjusting the transmitted spectrum.

Other features and advantages of the invention will be found in thedetailed description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a network configuration.

FIG. 2 is a block diagram of a communication system.

FIG. 3 is a block diagram of an encoder module.

FIG. 4 is a block diagram of a modulation module.

DETAILED DESCRIPTION

There are a great many possible implementations of the invention, toomany to describe herein. Some possible implementations that arepresently preferred are described below. It cannot be emphasized toostrongly, however, that these are descriptions of implementations of theinvention, and not descriptions of the invention, which is not limitedto the detailed implementations described in this section but isdescribed in broader terms in the claims.

As shown in FIG. 1, a network configuration 100 provides a sharedcommunication medium 110 for a number of communication stations102A-102E (e.g., computing devices, or audiovisual devices) tocommunicate with each other. The communication medium 110 can includeone or more types of physical communication media such as coaxial cable,unshielded twisted pair, power lines, or wireless channels for example.The network configuration 100 can also include devices such as bridgesor repeaters. The communication stations 102A-102E communicate with eachother using predetermined physical (PHY) layer and medium access control(MAC) layer communication protocols used by network interface modules106. The MAC layer is a sub-layer of the data link layer and provides aninterface to the PHY layer, according to the Open SystemsInterconnection (OSI) network architecture standard, for example. Thenetwork configuration 100 can have any of a variety of networktopologies (e.g., bus, tree, star, mesh).

The stations use an amplitude mask technique, described in more detailbelow, for managing the spectra of modulated signals without needing toexchange information among stations indicating which carriers are in useor disabled. The amplitude mask technique is used with a redundantcoding scheme that spreads data over multiple carriers so that thestation can control the spectrum of modulated signals with a highlikelihood that the modulated data can be recovered using redundantinformation.

In some implementations, the network interface modules 106 use protocolsthat include features to improve performance when the networkconfiguration 100 includes a communication medium 110 that exhibitsvarying transmission characteristics. For example, the communicationmedium 110 may include AC power lines in a house, optionally coupled toother media (e.g., coaxial cable lines).

Power-line communication systems use existing AC wiring to exchangeinformation. Owing to their being designed for much lower frequencytransmissions, AC wiring provides varying channel characteristics at thehigher frequencies used for data transmission (e.g., depending on thewiring used and the actual layout). To increase the data rate betweenvarious links, stations adjust their transmission parametersdynamically. This process is called channel adaptation. Channeladaptation results in adaptation information specifying a set oftransmission parameters that can be used on each link. Adaptationinformation includes such parameters as the frequencies used, theirmodulation, and the forward error correction (FEC) used.

The communication channel between any two stations provided by thecommunication medium 110 may exhibit varying channel characteristicssuch as periodic variation in noise characteristics and frequencyresponse. To improve performance and QoS stability in the presence ofvarying channel characteristics, the stations can synchronize channeladaptation with the frequency of the AC line (e.g., 50 or 60 Hz). Thereare typically variations in the phase and frequency of the AC line cyclefrom the power generating plant and local noise and load changes. Thissynchronization enables the stations to use consistent channeladaptation optimized for a particular phase region of the AC line cycle.An example of such synchronization is described in U.S. patentapplication Ser. No. 11/337,946, incorporated herein by reference.

Another aspect of mitigating potential impairments caused by the varyingchannel characteristics involves using a robust signal modulation formatsuch as OFDM. An exemplary communication system that uses OFDMmodulation is described below.

Any of a variety of communication system architectures can be used toimplement the portion of the network interface module 106 that convertsdata to and from a signal waveform that is transmitted over thecommunication medium. An application running on a station provides andreceives data to and from the network interface module 106 in segments.A “MAC Protocol Data Unit” (MPDU) is a segment of information includingoverhead and payload fields that the MAC layer has asked the PHY layerto transport. An MPDU can have any of a variety of formats based on thetype of data being transmitted. A “PHY Protocol Data Unit (PPDU)” refersto the modulated signal waveform representing an MPDU that istransmitted over the power line.

In OFDM modulation, data are transmitted in the form of OFDM “symbols.”Each symbol has a predetermined time duration or symbol time T_(s). Eachsymbol is generated from a superposition of N sinusoidal carrierwaveforms that are orthogonal to each other and form the OFDM carriers.Each carrier has a peak frequency ƒ_(i) and a phase Φ_(i) measured fromthe beginning of the symbol. For each of these mutually orthogonalcarriers, a whole number of periods of the sinusoidal waveform iscontained within the symbol time T_(s). Equivalently, each carrierfrequency is an integral multiple of a frequency interval Δƒ=1/T_(s).The phases Φ_(i) and amplitudes A_(i) of the carrier waveforms can beindependently selected (according to an appropriate modulation scheme)without affecting the orthogonality of the resulting modulatedwaveforms. The carriers occupy a frequency range between frequencies ƒ₁and ƒ_(N) referred to as the OFDM bandwidth. Referring to FIG. 2, acommunication system 200 includes a transmitter 202 for transmitting asignal (e.g., a sequence of OFDM symbols) over a communication medium204 to a receiver 206. The transmitter 202 and receiver 206 can both beincorporated into a network interface module 106 at each station. Thecommunication medium 204 represents a path from one station to anotherover the communication medium 110 of the network configuration 100.

At the transmitter 202, modules implementing the PHY layer receive anMPDU from the MAC layer. The MPDU is sent to an encoder module 220 toperform processing such as scrambling, error correction coding andinterleaving. Referring to FIG. 3, an exemplary encoder module 220includes a scrambler 300, a Turbo encoder 302, and an interleaver 304.

The scrambler 300 gives the information represented by the MPDU a morerandom distribution (e.g., to reduce the probability of long strings ofzeros or ones). In some implementations, the data is “XOR-ed” with arepeating Pseudo Noise (PN) sequence using a generator polynomial suchas:

S(x)=x ¹⁰ +x ³+1

The state bits in the scrambler 300 are initialized to a predeterminedsequence (e.g., all ones) at the start of processing an MPDU.

Scrambled information bits from the scrambler 300 can be encoded by anencoder that uses any of a variety of coding techniques (e.g.,convolutional codes). The encoder can generate a stream of data bits andin some cases auxiliary information such as one or more streams ofparity bits. In this example, the Turbo encoder 302 uses a Turbo code togenerate, for each block of m input information bits, a block of m “databits” (d) that represent the input information, a first block of n/2“parity bits” (p) corresponding to the information bits, and a secondblock of n/2 parity bits (q) corresponding to a known permutation of theinformation bits. Together, the data bits and the parity bits provideredundant information that can be used to correct potential errors. Thisscheme yields a code with a rate of m/(m+n).

The interleaver 304 interleaves the bits received from the Turbo encoder302. The interleaving can be performed, for example on blockscorresponding to predetermined portions of an MPDU. The interleavingensures that the redundant data and parity bits for a given block ofinformation are distributed in frequency (e.g., on different carriers)and in time (e.g., on different symbols) to provide the ability tocorrect errors that occur due to localized signal interference (e.g.,localized in time and/or frequency). The signal interference may be dueto a jammer or may be due to spectral shaping of the spectral shapingmodule 400 described below. The interleaving can ensure that theredundant information for a given portion of the MPDU is modulated ontocarriers that are evenly distributed over the OFDM bandwidth so thatlimited bandwidth interference is not likely to corrupt all of thecarriers. The interleaving can also ensure that the redundantinformation is modulated onto more than one symbol so that broadband butshort duration interference is not likely to corrupt all of the symbols.

The encoder module 220 includes a buffer that can be used to temporarilystore data and parity bits from the Turbo encoder 302, to be read out bythe interleaver 304 in a different order than the order in which theywere stored. For example, a buffer can include includes k “datasub-banks” of m/k bits each and k “parity sub-banks” of n/k bits each(e.g., the sub-banks can correspond to logical regions of memory). Inthe case of k=4, the data bits are divided into four equal sub-blocks ofm/4 bits, and the parity bits are divided into 4 equal sub-blocks of n/4bits (where both m and n are selected to be divisible by 4). The Turboencoder 302 writes the first m/4 data bits (in natural order) to thefirst data sub-bank, the next m/4 data bits to the second data sub-bank,and so on. The Turbo encoder 302 writes the first n/4 parity bits (innatural order) to the first parity sub-bank, the next n/4 parity bits tothe second parity sub-bank, and so on.

The interleaver 304 generates a stream of bits to be modulated ontocarriers of data symbols by reading from the sub-banks in apredetermined order. For example, the four data sub-banks of length m/4may be thought of as a matrix consisting of m/4 rows and four columns,with column 0 representing the first sub-bank, column 1 representing thesecond sub-bank, and so on. Groups of four bits on the same row (one bitfrom each sub-block) are read out from the matrix at a time, startingwith row 0. After a row has been read out, a row pointer is incrementedby StepSize before performing the next row read. After m/4/StepSize rowreads, the end of the matrix has been reached. The process is thenrepeated for different rows until all bits from the matrix have beenread out. The parity bits can be interleaved in a similar manner. Insome implementations, the data bits and the parity bits can alsointerleaved with each other in a predetermined manner.

In some modes of communication, called ROBO modes, the interleaver 304performs additional processing to generate increased redundancy in theoutput data stream. For example, ROBO mode can introduce furtherredundancy by reading each sub-bank location multiple times at differentcyclic shifts to represent each encoded bit by multiple bits at theoutput of the interleaver 304.

Other types of encoders and/or interleavers can be used that alsoprovide redundancy to enable each portion of an MPDU to be recoveredfrom fewer than all of the modulated carriers or fewer than all of themodulated symbols.

Referring again to FIG. 2, the encoded data is fed into a mapping module222 that takes groups of data bits (e.g., 1, 2, 3, 4, 6, 8, or 10 bits),depending on the constellation used for the current symbol (e.g., aBPSK, QPSK, 8-QAM, 16-QAM constellation), and maps the data valuerepresented by those bits onto the corresponding amplitudes of in-phase(I) and quadrature-phase (Q) components of a carrier waveform of thecurrent symbol. This results in each data value being associated with acorresponding complex number C_(i)=A_(i) exp(jΦ_(i)) whose real partcorresponds to the I component and whose imaginary part corresponds tothe Q component of a carrier with peak frequency ƒ_(i). Alternatively,any appropriate modulation scheme that associates data values tomodulated carrier waveforms can be used.

The mapping module 222 also determines which of the carrier frequenciesƒ₁, . . . , ƒ_(N) (or “tones”) within the OFDM bandwidth are used by thesystem 200 to transmit information according to a “tone mask.” Forexample, some carriers that are likely to interfere with licensedentities in a particular region (e.g., North America) can be avoided,and no power is radiated on those carriers. Devices sold in a givenregion can be programmed to use a tone mask configured for that region.The mapping module 222 also determines the type of modulation to be usedon each of the carriers in the tone mask according to a “tone map.” Thetone map can be a default tone map (e.g., for redundant broadcastcommunication among multiple stations), or a customized tone mapdetermined by a receiving station that has been adapted tocharacteristics of the communication medium 204 (e.g., for moreefficient unicast communication between two stations). If a stationdetermines (e.g., during channel adaptation) that a carrier in the tonemask is not suitable for use (e.g., due to fading or noise) the tone mapcan specify that the carrier is not to be used to modulate data, butinstead can use pseudorandom noise for that carrier (e.g., coherent BPSKmodulated with a binary value from a Pseudo Noise (PN) sequence). Fortwo stations to communicate, they should use the same tone mask and tonemap, or at least know what tone mask and tone map the other device isusing so that the signals can be demodulated properly.

A modulation module 224 performs the modulation of the resulting set ofN complex numbers (some of which may be zero for unused carriers)determined by the mapping module 222 onto N orthogonal carrier waveformshaving peak frequencies ƒ₁, . . . , ƒ_(N). The modulation module 224performs an inverse discrete Fourier transform (IDFT) to form a discretetime symbol waveform S(n) (for a sampling rate ƒ_(R)), which can bewritten as

$\begin{matrix}{{S(n)} = {\sum\limits_{i = 1}^{N}{A_{i}{\exp \left\lbrack {j\left( {{2\pi \; i\; {n/N}} + \Phi_{i}} \right)} \right\rbrack}}}} & {{Eq}.\mspace{14mu} (1)}\end{matrix}$

where the time index n goes from 1 to N, A_(i) is the amplitude andΦ_(i) is the phase of the carrier with peak frequency ƒ_(i)=(i/N) ƒ_(R),and j=√−1. In some implementations, the discrete Fourier transformcorresponds to a fast Fourier transform (FFT) in which N is a power of2.

A post-processing module 226 combines a sequence of consecutive(potentially overlapping) symbols into a “symbol set” that can betransmitted as a continuous block over the communication medium 204. Thepost-processing module 226 prepends a preamble to the symbol set thatcan be used for automatic gain control (AGC) and symbol timingsynchronization. To mitigate intersymbol and intercarrier interference(e.g., due to imperfections in the system 200 and/or the communicationmedium 204) the post-processing module 226 can extend each symbol with acyclic prefix that is a copy of the last part of the symbol. Thepost-processing module 226 can also perform other functions such asapplying a pulse shaping window to subsets of symbols within the symbolset (e.g., using a raised cosine window or other type of pulse shapingwindow) and overlapping the symbol subsets.

The modulation module 224 or the post-processing module 226 can includea spectral shaping module that further modifies the spectrum of a signalthat includes modulated symbols according to an “amplitude mask.” Whilethe tone mask can be changed by exchanging messages among stations in anetwork, the amplitude mask enables a station to attenuate powertransmitted on certain carriers without needing to exchange messagesamong the stations. Thus, the spectral shaping module enables dynamicspectral shaping in response to dynamic spectral constraints by changingthe amplitude of carriers that may cause interference. In some cases,the spectral shaping module sets the amplitude of the frequencycomponent below a predetermined limit in response to an event such asdetecting a transmission from a licensed entity.

Referring to FIG. 4, an exemplary implementation of the modulationmodule 224 includes a spectral shaping module 400 coupled to an IDFTmodule 402. The spectral shaping module 400 modifies the amplitude A_(i)for the carriers that are to be attenuated, providing an attenuatedamplitude A′_(i) to the IDFT module 402. The value of the phase andΦ_(i) for the attenuated carriers can be passed through the spectralshaping module 400 without modification. Thus, in this example, the IDFTmodule 402 performs a discrete Fourier transform that includes theattenuated carrier frequencies.

The amplitude mask specifies an attenuation factor α for the amplitudeA′_(i)=αA_(i) according to the amount by which the power is to beattenuated (e.g., 2 dB in amplitude for each 1 dB in power). Theamplitude A′_(i) is set below a predetermined amplitude that is normallyused for modulating the information (e.g., according to a predeterminedconstellation) such that the resulting radiated power does not interferewith other devices. The amplitude mask entry may also indicate that acarrier is to be nulled completely with the corresponding amplitude setto zero. The attenuated carriers are still processed by the receivingstation even if they are transmitted with zero amplitude so that themodulation and encoding scheme is preserved.

Generally, for two stations to communicate, they don't necessarily needto know what amplitude mask the other station is using (or whether thestation is using an amplitude mask at all). Even though no modificationof the modulation scheme between a transmitter and a receiver isnecessary to partially attenuate or fully attenuate (i.e., turn off) acarrier using the amplitude mask, in some cases, when a receivingstation updates a tone map (which determines how carriers within thetone mask are to be modulated) the receiving station will detect a verypoor signal-to-noise ratio on the attenuated carriers and may excludethem from the updated tone map (indicating that those carriers are notto be used for modulating data).

In alternative implementations, the spectral shaping module can beincluded in the post-processing module 226, for example, as aprogrammable notch filter that reduces the amplitude of one or morenarrow frequency bands in the signal.

An Analog Front End (AFE) module 228 couples an analog signal containinga continuous-time (e.g., low-pass filtered) version of the symbol set tothe communication medium 204. The effect of the transmission of thecontinuous-time version of the waveform S(t) over the communicationmedium 204 can be represented by convolution with a function g(τ;t)representing an impulse response of transmission over the communicationmedium. The communication medium 204 may add noise n(t), which may berandom noise and/or narrowband noise emitted by a jammer. At thereceiver 206, modules implementing the PHY layer receive a signal fromthe communication medium 204 and generate an MPDU for the MAC layer. AnAFE module 230 operates in conjunction with an Automatic Gain Control(AGC) module 232 and a time synchronization module 234 to providesampled signal data and timing information to a discrete Fouriertransform (DFT) module 236. After removing the cyclic prefix, thereceiver 206 feeds the sampled discrete-time symbols into DFT module 236to extract the sequence of N complex numbers representing the encodeddata values (by performing an N-point DFT). Demodulator/Decoder module238 maps the complex numbers onto the corresponding bit sequences andperforms the appropriate decoding of the bits (including deinterleaving,error correction, and descrambling). The data that was modulated ontocarriers that were subsequently attenuated by the spectral shapingmodule 400 can be recovered due to the redundancy in the encodingscheme.

Any of the modules of the communication system 200 including modules inthe transmitter 202 or receiver 206 can be implemented in hardware,software, or a combination of hardware and software.

Many other implementations of the invention other than those describedabove are within the invention, which is defined by the followingclaims.

1. A method for operating a communicating station in order to transmitinformation through a communication medium using a set of channels,wherein the channels have respective carrier frequencies, the methodcomprising: determining a list of one or more of the carrier frequenciesover which transmissions are to be attenuated; encoding information bitsaccording to an error correction code in order to obtain coded bits,wherein the coded bits represent the received information bits withredundancy; interleaving the coded bits to obtain interleaved bits;mapping blocks of the interleaved bits to corresponding sets of complexnumbers, wherein each set of complex numbers includes one complex numberfor each of the carrier frequencies; modifying each set of complexnumbers by attenuating one or more of the complex numbers in the setthat correspond to the one or more carrier frequencies on the list,wherein said attenuating includes scaling the one or more complexnumbers so that their amplitudes are less than a predetermined amplitudelevel; computing an inverse Fourier transform of each modified set ofcomplex numbers to obtain a sequence of time-domain samples; andtransmitting a time-domain signal over the communication medium based onthe sequence of time-domain samples.
 2. The method of claim 1, whereinthe one or more carrier frequencies on the list are determined withoutnegotiating with any other communicating station over the communicationmedium.
 3. The method of claim 1, wherein the one or more carrierfrequencies on the list are determined without exchanging messages withany other communicating station over the communication medium.
 4. Themethod of claim 1, wherein the list is not provided to a receivingstation that receives and demodulates the time-domain signal.
 5. Themethod of claim 1, wherein the predetermined amplitude level is zero. 6.The method of claim 1, wherein the predetermined amplitude level is anamplitude level that based on a regulatory limit for radiated power. 7.The method of claim 1, wherein the predetermined amplitude level is alevel that ensures non-interference with one or more other communicationstations.
 8. The method of claim 1, wherein the said interleavingspreads coded bits across different blocks of the interleaved bits. 9.The method of claim 1, wherein said interleaving spreads coded bits overthe carrier frequencies.
 10. The method of claim 9, wherein saidinterleaving spreads coded bits uniformly over the carrier frequencies.11. The method of claim 1, further comprising detecting transmissionfrom another communication station over the communication medium,wherein said determining the list includes adding one or more of thecarrier frequencies to the list in response to detecting saidtransmission.
 12. The method of claim 1, wherein said mapping of blocksof the interleaved bits to corresponding sets of complex numbersincludes applying a tone mask to avoid one or more frequencies specifiedby the tone mask, wherein one or more complex numbers in each set thatcorrespond to the one or more specified frequencies are set to zero andare not determined by the interleaved bits, wherein the tone mask isdetermined by negotiation with another communication station over thecommunication medium.
 13. The method of claim 1, further comprising:scrambling the information bits prior said encoding the informationbits.
 14. A communicating station for transmitting information through acommunication medium using a set of channels, wherein the channels haverespective carrier frequencies, the system comprising: memory forstoring a list of one or more of the carrier frequencies over whichtransmissions are to be attenuated; an encoding unit configured toencode information bits according to an error correction code in orderto obtain coded bits, wherein the coded bits represent the receivedinformation bits with redundancy; an interleaver configured tointerleave the coded bits to obtain interleaved bits; a modulationmodule configured to: map blocks of the interleaved bits tocorresponding sets of complex numbers, wherein each set of complexnumbers includes one complex number for each of the carrier frequencies;modify each set of complex numbers by attenuating one or more of thecomplex numbers in the set that correspond to the one or more carrierfrequencies on the list, wherein said attenuating includes scaling theone or more complex numbers so that their amplitudes are less than apredetermined amplitude level; and compute an inverse Fourier transformof each modified set of complex numbers to obtain a sequence oftime-domain samples.
 15. The communicating station of claim 14, whereinthe transmitter is configured to determine the one or more carrierfrequencies on the list without negotiating a change in modulationscheme with a receiving station over the communication medium.
 16. Thecommunicating station of claim 14, wherein the transmitter is configuredto determine the one or more carrier frequencies on the list withoutexchanging messages with any other communicating station over thecommunication medium.
 17. The communicating station of claim 14, whereinthe list is not provided to a receiving station that receives anddemodulates the time-domain signal.
 18. The communicating station ofclaim 14, wherein the predetermined amplitude level is zero.
 19. Thecommunicating station of claim 14, wherein the predetermined amplitudelevel is an amplitude level that based on a regulatory limit forradiated power.
 20. The communicating station of claim 14, wherein thepredetermined amplitude level is a level that ensures non-interferencewith one or more other communication stations.
 21. The communicatingstation of claim 14, wherein the interleaver is configured to spreadcoded bits across different blocks of the interleaved bits.
 22. Thecommunicating station of claim 14, wherein said interleaver isconfigured to spread coded bits over the carrier frequencies.
 23. Thecommunicating station of claim 22, wherein said interleaver isconfigured to spread coded bits uniformly over the carrier frequencies.24. The communicating station of claim 14, further comprising areceiver, wherein the communicating station is configured to add one ormore of the carrier frequencies to the list in response to thereceiver's detecting of a transmission from a licensed entity.
 25. Thecommunicating station of claim 14, wherein said modulation module isconfigured to apply a tone mask to avoid one or more frequenciesspecified by the tone mask, wherein one or more complex numbers in eachset that correspond to the one or more specified frequencies are set tozero and are not determined by the interleaved bits, wherein the tonemask is determined by negotiation with another communication stationover the communication medium.
 26. The communicating station of claim14, further comprising: a scrambler configured to scramble theinformation bits prior said encoding the information bits.
 27. Thecommunicating station of claim 14 further comprising: a front endconfigured to transmit a time-domain signal over the communicationmedium based on the sequence of time-domain samples.